1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic vehicle transmission, and more particularly to a control of a servo-actuators in a hydraulic control system for a vehicle transmission.
2. Description of the Prior Art
A conventional hydraulic control system for an automatic vehicle transmission includes hydraulically operated servo-actuators for effecting engagement or disengagement of friction devices such as clutches and brakes. In order to supply the actuators with an appropriate hydraulic pressure, the system is provided with a pressure regulating valve which produces a regulated line pressure. The line pressure is introduced through a control valve to the actuator to operate the same. In order to reduce any possible gear shift shock which may be produced when the servo-actuator is engaged, the hydraulic line leading to the actuator is provided with a pressure accumulator so that the hydraulic pressure which is to be introduced into the actuator is temporarily stored in the accumulator and applied to the servo-actuator. With this arrangement, it is expected that the pressure in the actuator is gradually increased to thereby effect a smooth engagement of the friction device which is associated with the servo-actuator. An example of such hydraulic control system is disclosed by the Japanese laid-open utility model 61-1769.
It has been experienced in the conventional system that the gear shift shock cannot be satisfactorily suppressed under certain circumstances. Since the pressure accumulator is sensitive to the fluidity of the hydraulic liquid, the rate of increase in the pressure in the actuator can change depending on the temperature of the hydraulic liquid and the gear shift shock may be produced under a condition where the pressure change can occur relatively quickly. In order to make the rate of pressure increase relatively slow irrespective of the temperature in the hydraulic liquid, the pressure accumulator may be of a large capacity. However, this solution is not recommendable because the accumulator of a larger capacity will make the transmission bulky. It should further be noted that in order to compensate for any possible manufacturing error in the servo-actuator in taking measures for suppressing the gear shift shock, the accumulator will have to be of different capacity. However, it is practically impossible to select an accumulator which is matched with the particular actuator mounted on each product.
It may be possible to design the pressure regulating valve so that the pressure regulating valve produces the hydraulic pressure which is suitable for the initial period of engagement of the servo-actuator. However, such design of the pressure regulator causes an increase in the range of pressure regulation so that precise pressure regulation will become difficult.